2. Set the price difference for this attribute in Sku Info Tab, and click Save button to save Sku.

In this case, the regular price is $35.00, the web price adds $10.00. So the final web price will show $45.00. You can also set a negative amount to this box. Eg: if the web price adds -10.00, the final web price will show $25.00

Alternatively, to setup on sale online only, you can setup sale price with Web Store Only option: